Surprise discovery: For most animals, sperm quality does not reduce with age

The team, led by the University of Oxford, assessed the results of 379 studies which covered a wide range of animals, including mammals, insects, birds, and fish. This found that, overall, advancing age seemed to have no impact on ejaculate size, number of sperm, sperm motility, or sperm viability.

For some animals, age even had a positive effect on reproductive traits. In many insect species, for example, ejaculate size, quantity of sperm, and sperm viability all improved with age. However, it is possible that this result may be explained by methodological limitations in the studies. For instance, in many insect studies, males were kept as virgins until old age, during which they are likely to accumulate sperm.

From all animals examined, only lab rodents showed age-related deterioration in most ejaculate traits measured. Dr Regina Vega-Trejo (Department of Biology, University of Oxford), co-lead author of the study, said: 'The lack of consistency across animals in the impact of aging on ejaculate traits suggests the jury is still out on whether age-related deterioration in ejaculates happens universally in animals or not'.

According to the researchers, the sharp contrast in ejaculate ageing patterns between most animals and humans might be because humans now live much longer than they did just a few centuries ago. This may have led to men now living beyond the age at which they evolved to maintain sperm function. Co-lead author Krish Sanghvi (Department of Biology, University of Oxford), said: 'The surprising differences between animals and humans could reflect stronger selection pressures for animals to maintain sperm function across all ages.'

The researchers also investigated whether age impacted reproductive outcomes (such as fertilisation success and fecundity) in the males of these species. Similar to ejaculate traits, they found that, overall, reproductive outcomes did not improve or decline with advancing age, which is again in stark contrast to what is seen in humans.

According to the authors, the results may be partly explained by methodological limitations, such as studies rarely testing animals that are at an advanced age for their species. They also suggest that deterioration in male fertility might not be occurring for each ejaculate trait in every species.

Senior author Dr Irem Sepil (Department of Biology, University of Oxford), said: 'Reproductive deterioration is of interest to a broad range of disciplines, from evolutionary ecology and demography to humanities and medicine. Our study challenges a key belief in these fields that male reproductive deterioration is universal, and documents where we may expect this to occur and why.'